The present invention relates generally to a fuel injection amount computing unit for an internal combustion engine, and more particularly, to a fuel injection amount computing unit which computes a fuel injection amount in accordance with an intake air flow.
One of previously proposed fuel injection amount computing units for an internal combustion engine is disclosed, for example, in JP-A 290939. This computing unit calculates, from an intake air flow Q measured by an airflow meter which is disposed in an engine suction system and an engine speed N, a basic fuel injection amount Tp.sub.0 =K.multidot.Q/M, wherein N is a constant. In order to avoid an influence of suction surging, the basic fuel injection amount is smoothed by surging smoothing means which determines a degree of smoothing in accordance with engine operating conditions.
Smoothing is carried out according to the following formula, obtaining a smoothed basic fuel injection amount Tp.sub.REAL. ND is a surging smoothing index for indicating a degree of smoothing; in the stationary state, ND=1 (1/2 shifted weighted average); in the transient state, ND=0 (1/1 shifted weighted average; smoothing prohibited) and; in the fully open state, ND=3 (1/8 shifted weighted average). EQU Tp.sub.REAL =[(2.sup.ND -1)Tp.sub.REAL +Tp.sub.0 ]/2.sup.ND
Additionally, the computing unit comprises phase adjustment means for correcting the basic fuel injection amount so as to correct a time lag produced from a measuring position of an intake air flow by the airflow meter to the cylinder or delay a phase in response to a boost, and a prefetched correction means for correcting the basic fuel injection amount based on a variation of the intake air flow calculated from a throttle valve opening degree and an engine speed so as to correct a measurement lag of the intake air flow in the initial stage of acceleration. The computing unit carries out the following correction, obtaining a final basic fuel injection amount AvTp: EQU AvTp=AvTp (1-Fload)+TrTp.multidot.Fload+ThsTp
In this formula, first and second terms of a right side correspond to the correction to be carried out by the phase adjustment means, TrTp being a trimmed basic fuel injection amount which is obtained by multiplying Tp.sub.REAL by a factor Ktrm (TrTp=Tp.sub.REAL .multidot.Ktrm) so as to correct a dispersion proper to a type of the engine, and Fload being a weighted average factor which is set between 0 and 1. A third term of the right side corresponds to a correction to be carried out by the prefetched correction means, TshTp being a prefetched correction amount which is set by a variation of the intake air flow calculated from the throttle valve opening degree and the engine speed.
As to such a known fuel injection amount computing unit for an internal combustion engine, however, there arises a problem in a characteristic upon acceleration. That is, referring to FIG. 6, if switching from the surging smoothing index ND=0 corresponding to the transient state to the surging smoothing index ND=3 corresponding to the fully open state is carried out on a condition of AvTp&gt;TrTp (or AvTp&gt;Tp.sub.REAL), this condition is established in a part of the preferred correction or at a timing as indicated by reference character A in FIG. 6, so that the surging smoothing index ND is switched to a value for the fully open state from the initial state of the transient state, carrying out needless surging smoothing operation, resulting in a deteriorated transient responsibility.
It is, therefore, an object of the present invention to provide a fuel injection amount computing unit for an internal combustion engine which provides an excellent transient responsibility with needless surging smoothing operation eliminated.